The present disclosure is directed to a device for directional solidification of a cast part. More particularly, this disclosure relates to a directional solidification casting device that controls a magnetic field to provide a desired microstructure.
A directional solidification (DS) casting process is utilized to orient crystal structure within a cast part. The desired orientation is provided by moving a mold from a hot zone within a furnace into a cooler zone at a desired rate. As the mold moves into the cooler zone, the molten material solidifies along a solidification front traveling in one direction.
Mixing of the molten material at the solidification front within the cast component is known to be deleterious to the quality of single crystal castings. Such mixing can be induced in the molten metal material by a magnetic field generated from an energized coil encircling the furnace cavity. Typically, an induction withdrawal furnace utilizes such an electric coil that produces energy required for maintaining the metal in a molten state. A susceptor is utilized to transduce an electromagnetic field produced by the electric coil into radiant heat transferred to the casting mold
The susceptor is usually a monolithic graphite cylinder located internal to the induction coil and external to the mold. The susceptor is heated by induction coils and radiates heat toward the mold to maintain metal in a molten state, and is intended to isolate the electromagnetic field from the hot zone of the furnace. The susceptor therefore fulfills two roles: it transduces the electromagnetic field from the primary induction coil into heat, and simultaneously attenuates said field.
Casting single crystal gas turbine parts can experience less than 100% yields. Some defects that occur during the casting process are separately nucleated grains, freckels, porosity, mis-oriented boundaries, and others. The causes of these defects are not always known, but have been empirically determined to be influenced by the geometry of the part and the relative orientation of the part and the mold in the furnace. It is hypothesized that remnant magnetic field in the interior of the susceptor may be detrimental to the production of the desired microstructure in a cast part. Calculations have been made estimating the significance for a given production furnace design. For a monolithic graphite susceptor, the thickness required to fully attenuate said electromagnetic field may be impractical for design and/or production considerations.
It has been recognized that the leakage of the magnetic field into the furnace hot zone could directly influence the solidification process during casting.